gzip size of modules should be calculated in bulk

[connorjclark]

impl:

source-map-explorer/src/explore.ts

Lines 265 to 280 in f408a40

	const getSize = options.gzip ? gzipSize.sync : Buffer.byteLength;
	mappingRanges.forEach((lineRanges, lineIndex) => {
	const line = lines[lineIndex];
	const mergedRanges = mergeRanges(lineRanges);
	mergedRanges.forEach(({ start, end, source }) => {
	const rangeString = line.substring(start, end + 1);
	const rangeByteLength = getSize(rangeString);
	if (!files[source]) {
	files[source] = { size: 0 };
	}
	files[source].size += rangeByteLength;

My concern is that calculating the gzip size of each mapping in isolation misses out on compression savings from common patterns. I expect this means that --gzip overestimates the gzip size of each module. Though I'm not sure what mergeRanges does, perhaps it is grouping everything?

(came up in GoogleChrome/lighthouse#10476)

[nikolay-borzov]

I expected this kind of issue. Getting 100% precise module gzip size seems almost impossible - a module is just part of a file. And gzip applies to the whole file. I still wonder what use of knowing the gzip size of a module when it's bundled with other modules.
Merging range purpose is to collect the biggest string that is part of a module. I haven't tested it enough but I think in most cases it should include the whole module code.
You can see actual bundle gzip size in totalBytes property

[connorjclark]

I still wonder what use of knowing the gzip size of a module when it's bundled with other modules.

Our specific use case is we want to minify the download time for a bundle. Because of gzip, a large JSON module is probably fewer bytes than an equivalently sized JS module. It's nice to get a rough sense (understanding that exact numbers are impossible) so that we know where to invest engineering effort.

Merging range purpose is to collect the biggest string that is part of a module. I haven't tested it enough but I think in most cases it should include the whole module code.

if this is true then I think this tool is close enough. I see a comment saying it combines continuous mappings of the same source - but I think mappings tend to be continuous for an entire module so that sounds good to me.